Estimation of the real aperture radar modulation transfer function directly from synthetic aperture radar ocean wave image spectra without a priori knowledge of the ocean wave height spectrum

Abstract

The phase and amplitude of the real aperture radar (RAR) modulation transfer function (MTF) are, applying both simulated and real synthetic aperture radar (SAR) image spectra, shown to strongly influence the SAR ocean wave imaging of range‐ (or near‐range) traveling wave systems. Conventionally, in situ measurement of the sea state has been used in connection with SAR estimation of the RAR MTF. In most cases, the SAR imaging has been simulated by varying the phase and amplitude of the transfer function until some criterium for best fit between the measured and simulated spectra is met. The main problem with this method is the need for in situ buoy measurements of the underlying ocean wave height spectrum. This paper proposes a new method for estimating the RAR MTF directly from the SAR ocean wave image spectrum. Hence the method differs from previously used methods in that it is independent of in situ measurements of the sea state. The only (weak) restriction is that the observed wave system is range‐ or near‐range traveling. On the basis of three range‐going profiles the RAR MTF phase and amplitude are estimated. Investigations using synthetic data reveal that the SAR image spectrum for realistic sea states is colored by the unknown transfer function to such an extent that the underlying wave spectral form is not critical. Experimentally, the phase and amplitude of the RAR modulation are computed using the Norwegian Continental Shelf Experiment 1988 data. It is shown that the phase is most important for the SAR spectral distribution. Typically, the phase is observed to be in the interval from 60° to 110° and the amplitude to be of the order of 10–18. Furthermore, it is shown from simulation studies that marked changes in real SAR image spectra crossing an atmospheric front are recreated when the measured MTF phase and amplitude are used. Eventually, the hydrodynamic modulation is also extracted from the RAR MTF data. Variations of the hydrodynamic MTF phase across the abovementioned front are focused on. The estimates confirm a consistent wind direction induced modulation on each side of the front. No marked trends are observed for the amplitude. The overall conclusion of the study is that the conformity between simulated and measured spectra is improved when measured RAR MTFs are incorporated in SAR imaging simulation procedures.